All were destroyed for security reasons on Churchill’s orders after the war. This is a replica, built by 60 volunteers, which was fired up last Tuesday.

At 61⁄2ft tall and running on no more power than a kettle, the Bombe could unravel 158 trillion possible combinations to unlock a seemingly random series of letters sent by the Nazis to the front lines, which were, in fact, highly complex codes, changing daily. Typewriter-like Enigma machines scrambled the letters using three or four rotor wheels. …